1. Field of the Invention
The present invention relates to the field of free-space optical communications.
2. Related Art
Throughput in current free-space optical communications systems is limited by the number of channels that can be transmitted and detected at any given time. In particular, throughput for quantum cryptography is very slow because each channel has an average intensity of one photon per pulse, and the rate at which such pulses can practically be generated is necessarily constrained by the rate at which photons can be detected.
Current methods of increasing the throughput of free-space optical systems are prohibitive in various ways. Traditional uniplex systems, which allow for transmission of only one channel per system, can be accumulated so that multiple signals are transmitting simultaneously. This is cost and space prohibitive, however, because it requires a complete transmitter and receiver set for each channel.
Wavelength division multiplexing (“WDM”) is another method of increasing the throughput of laser communications. In WDM, data is encoded on light sources having different wavelengths. The light sources of different wavelengths are then combined into a cohesive output beam. Upon reception, the beam can be separated using wavelength filters. Although WDM allows for some increase in data throughput, the increase is limited. As the number of wavelengths increases, a phenomenon called “cross-talk” begins to occur. Beams whose wavelengths are close to one another begin to bleed together. Once cross-talk occurs between two beams, the data carried by those beams becomes corrupt.
What is needed is a method of multiplexing in a free-space optical communications system that is scalable to a large number of simultaneous transmissions. What is also needed is a method of multiplexing that is capable of being implemented in a quantum cryptography system.